fix: address review feedback on persona feature

MAJOR fixes:
- Remove external YAML dependency (github.com/goccy/go-yaml)
  Per project convention: Go standard library only, zero dependencies.
  Convert all persona files from YAML to JSON format.
- Fix TestValidateWorkspacePath error expectation
  Go 1.21+ filepath.Join normalizes absolute paths differently.

MINOR fixes:
- Remove custom contains helper in persona_test.go (use strings.Contains)
- Add Unicode-safe CapitalizeFirst function for header titles
- ListBuiltinPersonas returns empty slice instead of nil on error
- Fix test comment about filepath.Join behavior

Documentation:
- Update README to reflect JSON-only persona format
- Update design doc with note about JSON decision
- Fix action.yml description for persona-file input

review/persona_test.go

@@ -87,26 +87,22 @@ func TestListBuiltinPersonas(t *testing.T) {
 	}
 }
 
-func TestLoadPersonaFromYAMLFile(t *testing.T) {
+func TestLoadPersonaFromJSONFile(t *testing.T) {
 	dir := t.TempDir()
-	path := filepath.Join(dir, "test.yaml")
+	path := filepath.Join(dir, "test.json")
 
-	content := `
-name: test
-display_name: Test Persona
-identity: |
-  You are a test persona.
-  Multi-line identity works.
-focus:
-  - testing
-  - validation
-ignore:
-  - nothing
-severity:
-  major: Big problems
-  minor: Small problems
-  nit: Tiny problems
-`
+	content := `{
+		"name": "test",
+		"display_name": "Test Persona",
+		"identity": "You are a test persona.\nMulti-line identity works.",
+		"focus": ["testing", "validation"],
+		"ignore": ["nothing"],
+		"severity": {
+			"major": "Big problems",
+			"minor": "Small problems",
+			"nit": "Tiny problems"
+		}
+	}`
 
 	if err := os.WriteFile(path, []byte(content), 0644); err != nil {
 		t.Fatalf("failed to write test file: %v", err)
@@ -131,59 +127,25 @@ severity:
 	}
 }
 
-func TestLoadPersonaFromJSONFile(t *testing.T) {
-	dir := t.TempDir()
-	path := filepath.Join(dir, "test.json")
-
-	content := `{
-		"name": "test",
-		"display_name": "Test Persona",
-		"identity": "You are a test persona.",
-		"focus": ["testing"],
-		"ignore": ["nothing"],
-		"severity": {
-			"major": "Big problems",
-			"minor": "Small problems",
-			"nit": "Tiny problems"
-		}
-	}`
-
-	if err := os.WriteFile(path, []byte(content), 0644); err != nil {
-		t.Fatalf("failed to write test file: %v", err)
-	}
-
-	p, err := LoadPersona(path)
-	if err != nil {
-		t.Fatalf("LoadPersona failed: %v", err)
-	}
-
-	if p.Name != "test" {
-		t.Errorf("Name = %q, want %q", p.Name, "test")
-	}
-	if p.DisplayName != "Test Persona" {
-		t.Errorf("DisplayName = %q, want %q", p.DisplayName, "Test Persona")
-	}
-}
-
 func TestLoadPersonaValidation(t *testing.T) {
 	tests := []struct {
 		name    string
-		yaml    string
+		json    string
 		wantErr string
 	}{
 		{
 			name:    "missing name",
-			yaml:    "identity: test",
+			json:    `{"identity": "test"}`,
 			wantErr: "name is required",
 		},
 		{
 			name:    "missing identity",
-			yaml:    "name: test",
+			json:    `{"name": "test"}`,
 			wantErr: "identity is required",
 		},
 		{
 			name: "display_name defaults to name",
-			yaml: "name: test\nidentity: test identity",
+			json: `{"name": "test", "identity": "test identity"}`,
 			// No error expected - should succeed
 		},
 	}
@@ -191,8 +153,8 @@ func TestLoadPersonaValidation(t *testing.T) {
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
 			dir := t.TempDir()
-			path := filepath.Join(dir, "test.yaml")
-			if err := os.WriteFile(path, []byte(tt.yaml), 0644); err != nil {
+			path := filepath.Join(dir, "test.json")
+			if err := os.WriteFile(path, []byte(tt.json), 0644); err != nil {
 				t.Fatalf("failed to write test file: %v", err)
 			}
 
@@ -222,21 +184,56 @@ func TestLoadPersonaValidation(t *testing.T) {
 }
 
 func TestLoadPersonaFileNotFound(t *testing.T) {
-	_, err := LoadPersona("/nonexistent/path/persona.yaml")
+	_, err := LoadPersona("/nonexistent/path/persona.json")
 	if err == nil {
 		t.Error("expected error for nonexistent file")
 	}
 }
 
-func TestLoadPersonaInvalidYAML(t *testing.T) {
+func TestLoadPersonaInvalidJSON(t *testing.T) {
 	dir := t.TempDir()
-	path := filepath.Join(dir, "invalid.yaml")
-	if err := os.WriteFile(path, []byte("not: valid: yaml: here"), 0644); err != nil {
+	path := filepath.Join(dir, "invalid.json")
+	if err := os.WriteFile(path, []byte("not valid json {"), 0644); err != nil {
 		t.Fatalf("failed to write test file: %v", err)
 	}
 
 	_, err := LoadPersona(path)
 	if err == nil {
-		t.Error("expected error for invalid YAML")
+		t.Error("expected error for invalid JSON")
+	}
+}
+
+func TestCapitalizeFirst(t *testing.T) {
+	tests := []struct {
+		input string
+		want  string
+	}{
+		{"hello", "Hello"},
+		{"Hello", "Hello"},
+		{"HELLO", "HELLO"},
+		{"a", "A"},
+		{"", ""},
+		{"日本語", "日本語"}, // Non-ASCII: Japanese doesn't have case
+		{"über", "Über"},   // German umlaut
+		{"élève", "Élève"}, // French accent
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.input, func(t *testing.T) {
+			got := CapitalizeFirst(tt.input)
+			if got != tt.want {
+				t.Errorf("CapitalizeFirst(%q) = %q, want %q", tt.input, got, tt.want)
+			}
+		})
+	}
+}
+
+func TestListBuiltinPersonasReturnsEmptySlice(t *testing.T) {
+	// ListBuiltinPersonas should return an empty slice (not nil) on error.
+	// We can't easily test the error case, but we can verify the success case
+	// returns a proper slice.
+	names := ListBuiltinPersonas()
+	if names == nil {
+		t.Error("ListBuiltinPersonas should return empty slice, not nil")
 	}
 }